used for removing the clear supernatant liquid from precipitates, or poisonous, bad-smelling liquids, etc. B. For titrating.-Burettes' for measuring the number 1 Stender's glass manufactory in Lampspringe furnishes graduated glass vessels with graduation in red burned in with enamel. König's Ventilburette in Dingler, ccxvii. 134. Kleinert's Chameleon-burette in Fresenius' Ztschr. 1878, p. 183. Bürettenstative in Dingler, ccxxii. 465; ccxxix. 366. Fresenius' Ztschr. 1877, p. 82, 228. Fig. 23. is standard solution which have been allowed to run into the assay fluid until the final reaction reached. For measuring assay and normal solutions, it is a very good plan to place two burettes in the same stand side by side. The burette represented by Fig. 22 (p. 47) is well adapted for all uses. It is provided with a glass cock a; b is a glass cap to protect the liquid from dust; cc' are openings in it for the admission of air. Mohr's burette is the simplest form of the apparatus, and has the preference over all others for general purposes. It is, however, not to be recommended in cases where the rubber of the pinch-cock will be liable to act chemically on the liquid employed. (Fig. 23.) 3. Assays by colorimetric analysis.-This method is based upon the principle that equal volumes of solutions of an equally intense color contain also equal quantities of coloring matter. By comparing fluids of an equal intensity of color, and taking the volume into consideration, a conclusion is formed as to the percentage of the coloring body which is contained in the one to be determined. The same manipulations occur here as in assays by gravimetric analysis, namely, solution, precipitation, etc., and in addition comparison of the colored assay solution with standard colored solutions contained in tubes or tapering glasses of known cross-sections, measuring the solutions in calibrated cylinders, etc. III. Assay Furnaces. 9. GENERAL REMARKS. The choice of an assay furnace will depend chiefly on the degree of heat to be obtained, and whether the substances are to be oxidized or reduced, or only calcined, fused, sublimed, or distilled. Furnaces, accordingly, are divided into muffle furnaces, draught or wind furnaces, blast furnaces, sublimation furnaces, and distillation furIn regard to their construction, they vary chiefly according to the fuel to be used (flaming or glowing fuel). naces. 10. MUFFLE-FURNACES.1 The principal part of this is the muffle (Fig. 24). It is usually made of refractory clay, sometimes, though rarely, of iron. It is open in front, and closed at the rear; and the semi-cylindrical body is often provided along the sides with draft orifices, as shown. It is either connected Fig. 24. Engin. and Min. Journ. 1878, No. 26, p. 443. Silliman, Double MuffleFurnace, 1876, vol. xxii. No. 17. with the bottom, or stands loose upon it. It serves for the reception of the assay charge, and is heated from the outside by a glowing or flaming fire. These furnaces are absolutely necessary for oxidizing processes (calcining, cupellation, refining), but they are also adapted for operations requiring only the production of a high temperature (glowing, reducing, and purifying fusion, etc.), that is to say, when only temperatures not exceeding the fusing point of gold and copper (about 1200° C., 2192° F.) are required (they are, therefore, not available for assays of cast iron). In the latter cases the fuel is not completely utilized, and besides, they are more difficult to attend than the wind and blast furnaces, where the crucibles, etc., are placed directly in the glowing fire, or come in direct contact with the flame. The furnaces are either bricked in (for instance, large muffle-furnaces for burning coal), or they are portable. In the latter case, the furnace for receiving the muffle is constructed of fire-clay which is sometimes surrounded with a casing of sheet iron (mint furnaces). The work connected with the muffle-furnace consists chiefly in heating it, regulating the temperature (by reducing or urging the fire, regulating the admission of air, opening or closing the mouth of the muffle, by removing or piling up fuel, etc.), in stirring the fire regularly (in doing this the fuel must be piled chiefly upon the front part of the grate and only a thin layer upon the back part), in ventilating the grate frequently, in repairing (that is, lining defective places in the walls of the furnace, filling in of cracks in the bottom of the muffle with fire-clay, or scraping the bottom and lining it by strewing it with powdered fireclay, cupel ashes, chalk, pounded assay vessels, etc.), introducing and removing the assay vessels in the muffle, cleansing the furnace after the work is finished by draw |